Industrial waste water containing dissolved hydrogen sulfide presents a significant pollution problem because of its high toxicity and unpleasant odor, even at low concentrations. The treatment of such waste water is necessary before discharging it to the environment to reduce the hydrogen sulfide content to acceptable levels. The present invention provides a simple and effective method of removing hydrogen sulfide from such waste water streams by the initial addition of a chelated heavy metal catalyst, e.g., iron chelate solution, and, thereafter, adding mostly chelate solution, to replace degraded chelon, while retaining nearly all of the heavy metal in solution (about 90% to about 100%, preferably 95% to 100% by weight).
Typical sour water streams are those produced in oil refineries by water washing of sour liquid hydrocarbons and various cooler and condenser surfaces. Condensation of geothermal steam also produces sour water which requires treatment. Although the present invention may be used for the treatment of any sour water stream regardless of its source, the invention is of particular significance for treating geothermal condensates.
In a geothermal power plant, geothermal steam is used to power a steam turbine which is connected to an electric power generator. The exhaust steam from the turbine is supplied to a condenser, and the resultant steam condensate is removed for reuse or discard. In prior art processes about 1/3 of the condensate is discarded, usually after cooling tower treatment. Geothermal steam contains dissolved hydrogen sulfide in amounts which may range, for example, from as low as about 5 ppm to as high as about 1600 ppm and typically may average about 150 ppm to 250 ppm. Dependent upon the type of condenser and its efficiency, a significant percentage, e.g., as much as 80%, of the hydrogen sulfide in the geothermal steam will end up as dissolved hydrogen sulfide in the condensate. This sour water stream must be treated to remove hydrogen sulfide in order to avoid environmental pollution.
U.S. Pat. No. 4,076,621 discloses a process for removing hydrogen sulfide from sour water by air stripping the dissolved hydrogen sulfide from the sour water and then scrubbing the air stream with an aqueous solution of chelated iron. U.S. Pat. Nos. 4,414,817; 4,451,442; and 4,468,929 disclose processes for removing hydrogen sulfide from geothermal steam or condensate using an aqueous solution containing at least the stoichiometric amount of a chelated polyvalent metal. U.S. Pat. No. 4,363,215 discloses a process for removing hydrogen sulfide from geothermal steam condensate using hydrogen peroxide and an iron chelate catalyst. U.S. Pat. Nos. 4,614,644 and 4,629,608 disclose processes for removing hydrogen sulfide from geothermal steam using a chelated iron solution and a cationic polymeric catalyst. U.S. Pat. Nos. 4,451,442; 4,468,929; and 5,057,292 disclose processes for removing hydrogen sulfide from geothermal steam condensate including loss of iron chelate catalyst that is replaced at the same rate of loss.
The known sour water treatment processes that rely on the use of chelated polyvalent metal solutions, e.g. chelated iron, are complex and have disadvantages, such as excessive consumption or discard of expensive polyvalent metal and chelating agent.
One method of treating geothermal steam condensate, currently being practiced, is shown in FIG. 1. In accordance with the prior art method shown in FIG. 1, sour condensate is processed by directing about 2/3 of the condensate to the top of a cooling tower, where most of the condensate is cooled and evaporated, and the remaining about 1/3 of the sour condensate is directed to deep well disposal, reinjecting the 1/3 of the condensate into the geological formation near its origin. Sufficient sour condensate is directed into the cooling tower to provide a slight excess of condensate that is received in a cooling tower collection tray or basin to ensure that sufficient condensate is available for recycle to the steam condenser. The excess condensate received in the cooling tower collection basin is directed to deep well disposal together with the portion of the condensate that is not directed to the cooling tower.
Polyvalent metal chelate catalyst solution, e.g., iron chelate solution, is required for removal of dissolved hydrogen sulfide contained in the sour steam condensate and the polyvalent metal chelate solution lost to deep well disposal can be added to the sour condensate either prior to the condensate entering the cooling tower, or in the cooling tower collection basin, as shown in FIG. 1.
In accordance with the present invention, at least 90% by weight of the polyvalent metal is retained for continuous recirculation, without more than 10%, and preferably less than 5% by weight loss of sour condensate and polyvalent metal to disposal, requiring only the addition of chelon to the process due to chelon degradation, without substantial losses of polyvalent metal. Some blowdown or removal of water and polyvalent metal from the cooling tower basin may be required, up to about 10% by weight, and preferably 0-5% discard of condensate, to limit the concentration of salts in the water recirculated to the condenser. It is understood that this blowdown (0-10%, preferably 0-5% of the condensate) will contain a small amount of fully chelated iron, which must be replaced by the addition of a small amount of fully chelated iron to the system. However, because the water lost by evaporation is replaced by condensation of steam, there should be little or no salts entering the system and it should be possible to operate with very little (0-10%, preferably 0-5%, and ideally 1-3%, e.g. 2%) blowdown.